This has only happened 3 times in about 2 weeks. Most the time I'm at a table so end up having the charger plugged in as I'm using the Pro 4 and there are no issues. I've noticed if I use the tablet when not plugged in and the battery drops to around 20% and I then stop using it and just put it to one side to recharge I can hear the fan come on after awhile. Just now I heard it and went to unplug the power, I noticed it was very warm and switched it on. There did not appear to be anything intensive running and battery was at 99% full. Ambient room temperature is 13°C. The Pro 4 was on a sofa so effectively had insulation under it. Pretty sure it was on the sofa the last 2 times it happened as well. I will of course no longer charge it on the sofa but is it normal to get warm when charging? I'm new to tablets so not sure if this is normal or not.

I've sometimes found that, when I leave it charging but don't manually put it to sleep, it will begin doing some system things which cause the fan to come on. I can see some high CPU usage when I immediately open Task Manager but it quickly goes away. I suspect that there are some background tasks that run when the system is idle and on charger.

Even in use while charging mine doesn't turn the fan on with basic use... Why don't you check the task manager for any apps that could be cycling the CPU. I saw I had Excel today! WOW! Excel with a simple sheet was eating up my CPU. Before that, it was Microsoft Skype 32 Bit which I got rid of very easily... Just make sure NO PROGRAMS are contributing to it!